1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling the air-fuel ratio of an internal combustion engine, and a recording medium storing a program for controlling the air-fuel ratio of an internal combustion engine.
2. Description of the Related Art
Heretofore, it has been known in the art to place an exhaust sensor downstream of a catalytic converter that is disposed in the exhaust passage of an internal combustion engine, the exhaust sensor having a sensitive element that is sensitive to a certain component of the exhaust gas, and control the air-fuel ratio of the exhaust gas that is supplied from the internal combustion engine to the catalytic converter in order to converge an output of the exhaust gas sensor to a predetermined target value for the purpose of achieving a desired exhaust gas purifying capability of the catalytic converter. For example, Japanese laid-open patent publication No. 11-324767 and U.S. Pat. No. 6,188,953 disclose a system, proposed by the applicant of the present application, wherein an O2 sensor serving as an exhaust gas sensor for generating an output depending on the concentration of oxygen in an exhaust gas is disposed downstream of a catalytic converter comprising a three-way catalyst, and the air-fuel ratio is controlled to converge the output of the O2 sensor to a predetermined target value for thereby enabling the catalytic converter to purify CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides) contained in the exhaust gas. The disclosed system is based on he phenomenon that when the air-fuel ratio of the exhaust gas supplied from the internal combustion engine to the catalytic converter is controlled at an air-fuel ratio state wherein the output (output voltage) of the O2 sensor disposed downstream of the catalytic converter is settled on a certain constant value, the purification rate of CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides) by the catalytic converter is kept at a good level (substantially maximum level) regardless of the deteriorated state of the catalytic converter.
Some exhaust gas sensors such as O2 sensors have a heater for heating the sensitive element thereof to quickly activate the sensitive element after the internal combustion engine starts to operate.
Generally, exhaust gas sensors such as O2 sensors have their output characteristics (representing an output voltage depending on the content of a certain component in the exhaust gas) that are variable depending on the temperature of the sensitive element. According to the findings of the inventors of the present invention, when the output of an O2 sensor disposed downstream of a catalytic converter varies depending on the temperature of the sensitive element of the O2 sensor, the output of the O2 sensor that achieves a desired exhaust gas purifying capability of the catalytic converter also varies. Therefore, in the case where the temperature of the sensitive element of the O2 sensor is easily variable due to the exhaust system layout or an operating state of the internal combustion engine, if the target value for the output of the O2 sensor is set to a constant value, then it tends to be difficult to sufficiently achieve a desired exhaust gas purifying capability of the catalytic converter even by controlling the air-fuel ratio to maintain the output of the O2 sensor at the target value thereof.